BMI Calculator

Free BMI calculator with Adult (20+) WHO categories and Child/Teen (2–19) CDC BMI-for-age percentiles. Enter height and weight in metric (cm/kg) or imperial (ft+in/lb). Adult mode shows BMI, category, healthy weight range, and an ideal-weight estimate (BMI ≈ 22). Child/Teen mode requires age and sex and reports percentile and z-score. Runs in your browser. Screening estimate only — not a diagnosis or medical advice.

How to Use the BMI Calculator

Choose Adult (WHO) or Child/Teen (CDC BMI-for-age), enter height and weight in metric or imperial units, then review BMI and category results. Screening estimate only — not medical advice.

  1. Select Adult (20+) or Child / Teen (2–19) under Calculation mode.
  2. Choose metric (cm, kg) or imperial (ft+in, lb).
  3. For Child/Teen: enter age in years (2–19), optional months (0–11), and sex (required).
  4. Enter height and weight.
  5. Press Calculate and review BMI plus adult WHO category or child/teen percentile, z-score, and CDC category.

Adult WHO categories

  • Underweight — BMI < 18.5
  • Normal — 18.5 ≤ BMI < 25
  • Overweight — 25 ≤ BMI < 30
  • Obese — BMI ≥ 30

Common mistakes

  • Applying adult WHO cutoffs to children or teens.
  • Mixing units without using the unit switcher.
  • Treating BMI as a diagnosis or full health assessment.

Limitations

BMI does not measure body fat, muscle mass, or overall health. Athletes, pregnancy, and some medical conditions can make BMI misleading. Child/Teen results require accurate age and sex. Screening estimate only — not medical advice.

Privacy

Height, weight, age, and sex stay in your browser. Values are not uploaded to ToolsLibrary servers for this calculator.

FAQ

What is BMI?

BMI (Body Mass Index) is a screening number from height and weight: weight in kilograms divided by height in metres squared. It is an estimate, not a full health assessment.

How is BMI calculated in this tool?

We use BMI = weight (kg) ÷ [height (m)]² after converting imperial inputs when needed. Adult results use WHO category cutoffs. Child/Teen mode then applies CDC BMI-for-age percentiles by age and sex.

Can I use kg and cm or lb and ft/in?

Yes. Metric uses centimetres and kilograms. Imperial uses feet plus inches and pounds. Switching units converts the values you already entered.

What do the adult BMI categories mean?

Underweight is BMI under 18.5, Normal is 18.5 to under 25, Overweight is 25 to under 30, and Obese is 30 or higher. These are WHO adult cutoffs used in Adult mode only.

What is Child/Teen BMI mode?

For ages 2–19, BMI is calculated the same way, then compared with CDC BMI-for-age charts by age and sex. You get a percentile, z-score, and categories such as Healthy Weight or Overweight. Adult WHO cutoffs are not used.

Does age or sex change adult BMI?

No. In Adult mode, age and gender are optional and do not change the BMI number. In Child/Teen mode, age and sex are required for BMI-for-age interpretation.

Is BMI a diagnosis?

No. BMI and BMI-for-age are screening estimates, not diagnoses or medical advice. Talk with a qualified clinician for personalized guidance.

What healthy weight range does adult mode show?

Adult mode shows a healthy weight range corresponding to BMI 18.5–24.9 for your height, plus an ideal-weight estimate near BMI ≈ 22. These are formula-based reference ranges, not personalized prescriptions.

Is my height and weight uploaded?

No. Calculations run in your browser. Height and weight are not uploaded to ToolsLibrary servers for this tool.

Why can BMI misclassify muscular people?

BMI uses only height and weight. Extra muscle raises weight without distinguishing fat from lean mass, so athletes can land in Overweight while still having low body fat. Treat BMI as a screening estimate, not a body-composition measurement.